The first object here is to obtain insight into the genetic determination of form. In bacteriophage T4, mutations are known that cause alteration of capsid length but not diameter. Others effect changes of both dimensions. The most extensively studied group consists of the former type, more specifically those mapping in gene 23 (major capsid structural protein). More than 50 independent mutations have all mapped at 8 gene-23 sites, and these are distributed in 4 tight clusters. We suggest that these clusters reflect 4 short segments of the protein whose interactions play decisive roles in controlling capsid length. Although wild-type T4 lysates rarely contain more than 1-2% capsids of abnormal length, stocks of gene-23 ptg mutants frequently contain more than 75%. We will compare the amino acid sequences of normal and mutant gene-23 proteins, expecting to learn what amino acid substitutions cause loss of fidelity in capsid length determination. A complete study of capsid-shape control must include investigating the other genes that play significant roles. Genes 22 and 24 are already known to have such functions, and genetic study of gene-22 mutants has been started. It will be followed by a molecular study. Identification of still unrecognized shape-controlling genes involves isolating gene-23 ptg revertants. Among them, those occurring at genetic sites different from the original mutation are selected. Mapping them will presumably identify genes whose products interact with gene-23 protein in length control. Our previous work led us to resurrect the partial replica hypothesis for T4 DNA replication. The hypothesis predicts the UVed T4 will replicate regions of the genome located near initiation sites independent of their rescue from the irradiated DNA molecule. Distant segments that are more frequently isolated from initiation sites by UV lesions should tend to remain unreplicated until after rescue. Our second aim is to test that prediction. Mutant T4 strains (imm), that allow delayed superinfection, when combined with use of multimarked helper phages will enable us to make such a test.